1. Field of the Invention
The present invention relates to a hydrogen sensor and a method for detecting hydrogen, the hydrogen sensor being able to simply detect hydrogen at room temperature and to detect only hydrogen without receiving any influence from the presence of flammable gases such as a methane gas and an alcohol.
2. Description of the Related Art
Hitherto, gas detection elements using metal oxide semiconductors such as SnO2 and In2O3 as a gas sensitive member have been known, and among the gas detection elements of this type, as an element which is devised so as to selectively detect a H2 gas, for example, a hydrogen gas detection element described in Japanese Unexamined Patent Application Publication No. 3-259736 has been known.
The hydrogen gas detection element disclosed in Japanese Unexamined Patent Application Publication No. 3-259736 has the structure in which an insulating substrate, an Sn oxide layer, comb-shaped electrodes, and a catalytic layer made of palladium are sequentially provided in that order on a heating element. In a final step of forming the above element, by performing heat treatment, besides improvement in sensitivity to a H2 gas, the sensitivity to gases other than a H2 gas is decreased. The hydrogen gas detection element described above has the structure in which comb-shaped Au electrodes are formed on a semiconductive Sn oxide film (SnO2), and on the films mentioned above, a catalytic layer of Pd (palladium) or Pt (platinum) formed by vacuum deposition and having a mass film thickness of approximately 1 to 5 nm is provided. The hydrogen gas detection element described above responds to an ethanol gas (C2H5OH), a propane gas (C3H8), and a carbon monoxide gas (CO) besides a hydrogen gas.
In addition, as an example of a hydrogen sensor having a different structure from the above structure, for example, the structure has been proposed in which on a surface of a gas sensitive member made of a metal oxide semiconductor such as SnO2 or In2O3, a thin film made of a non-flammable material such as Al2O3, SiO2, or Si3N4, is formed, the thin film preventing molecules other than hydrogen from permeating therethrough and easily permitting hydrogen molecules to permeate. A hydrogen sensor having the structure described above aims to realize a highly sensitive hydrogen selective sensor by forming a dense, uniform, continuous thin film (hydrogen selective permeable film) on a surface of a gas sensitive member so as to reduce interference caused by gases other than hydrogen.
Furthermore, as a hydrogen sensor having another structure, as disclosed in Japanese Unexamined Patent Application Publication No. 6-148112, the structure has been known in which comb-shaped electrodes of Pt, a thin-film gas sensitive member of In2O3, a powdered catalyst of Pd, and a partly poisoning agent of a Si oxide adhering to the exterior surface of the powdered catalyst are provided in that order on an insulating substrate to form a laminate structure. A hydrogen gas detection element disclosed in Japanese Unexamined Patent Application Publication No. 6-148112 is formed by sputtering on the surface of the gas sensitive member and that of the powdered catalyst, and in one example of the above Japanese Unexamined Patent Application Publication, the structure has been disclosed which is formed by the steps of forming comb-shaped electrodes of Pt having a mass film thickness of 500 Å and a catalyst of Pd having a mass film thickness of 10 Å on a gas sensitive member of In2O3 having a mass film thickness of 126 Å, and on these films thus formed, further forming a partly poisoning agent (mass film thickness of 10 to 12,000 Å).
Furthermore, in test results of the example and comparative example of Japanese Unexamined Patent Application Publication No. 6-148112, it has been disclosed that the sensitivity (change in resistance) to a H2 gas, a CO gas, and a C2H5OH gas is changed in accordance with the film thickness of the partly poisoning agent and the temperature of the element. In addition, as for the temperature of the element, the measurement was performed at temperatures from 60 to 300° C.
Furthermore, in Japanese Unexamined Patent Application Publication Nos. 2002-328108 and 2002-328109, a hydrogen sensor has been disclosed having the structure in which an insulating layer incorporating a heating element is provided on a silicon substrate, comb-shaped electrodes and a gas sensitive film are further provided thereon, and on this gas sensitive film, catalyst clusters and a partly poisoning agent are dispersed.
According to the hydrogen gas detection element disclosed in Japanese Unexamined Patent Application Publication No. 2002-328108, the structure has been disclosed in which an insulating layer incorporating a heater is laminated on a silicon substrate, a gas sensitive film of SnO2 or In2O3 or of an oxide semiconductor primarily composed thereof is formed on the above insulating layer, comb teeth-shaped electrodes are disposed at an interface between this gas sensitive film and the insulating layer thereunder, catalyst clusters of a noble metal such as Pd are dispersed on the upper surface side of the gas sensitive film, and a powdered partly poisoning agent of Si is dispersed on the upper surface of the gas sensitive film and the catalyst clusters at the surface side thereof. In addition, Japanese Unexamined Patent Application Publication No. 2002-328108 disclosed that TiO2, ZnO, Cu2O, NiO, FeO, WO3, and the like may be used as the oxide semiconductor forming the gas sensitive film. According to description of performance evaluation, in a sample in which catalyst clusters (thickness of 2 nm) of Pd and a partly poisoning agent (Si film thickness of 2, 20, 50 nm) of a Si oxide film are formed on a gas sensitive film (thickness of 7, 13, 20 nm) of SnO2, detection of a hydrogen gas can be efficiently performed in a state heated to 220° C. In Japanese Unexamined Patent Application Publication No. 2002-328109, in the structure similar to that in Japanese Unexamined Patent Application Publication No. 2002-328108, the invention has been disclosed in which the film thickness of the partly poisoning agent is specified in a particular range.
The conventional hydrogen gas sensors described in Japanese Unexamined Patent Application Publication Nos. 3-259736, 6-148112, 2002-328108, and 2002-328109 each have been well known as a sensor using the following basic principle. According to this basic principle, when the reducing gases (flammable gases) described above act on an oxide semiconductor material such as SnO2 or In2O3, since the reducing gases serve to pull oxygen out of the oxide semiconductor material, and hence electrons trapped with oxygen are allowed to remain in the semiconductor, the thickness of an electrical depletion layer is decreased, and a region in which charged carriers are present is increased, thereby causing the change in resistance. The technique described above relate to a technique in which the selectivity of hydrogen is improved as much as possible by decreasing influence of flammable gases other than hydrogen.
However, in consideration of the above basic operation principle, the hydrogen gas sensors having the conventional structure surely also react with reducing gases other than hydrogen, and hence a problem has occurred in that it is not possible to selectively detect only hydrogen. In addition, the hydrogen gas sensors of the conventional structure each have a problem in that after the hydrogen sensor once reacts with a hydrogen gas, in an environment other than an oxygen-containing environment such as the atmospheric environment or an oxygen gas environment, the resistance of the sensor is not allowed to return to an original level. For example, there has been a problem in that the sensors described above cannot be used when detection of hydrogen is to be continuously and repeatedly performed in an environment in which oxygen is not present, such as an inert gas environment.
Furthermore, since the hydrogen sensors having the conventional structure described above use a reaction between an oxide material and a reducing gas, the sensitivity tends to be improved at a high temperature, such as 200 to 300° C. Hence, a heater is generally incorporated in the hydrogen sensor so that detection operation is performed in a heated state, and in other words, this type of hydrogen sensor is difficult to be used at room temperature. In addition, for detection operation, since the oxide material must be heated to a high temperature, electricity must be applied to the heater, and as a result, electric power consumption is disadvantageously increased.
In addition, in the hydrogen sensors having the conventional structure described above, after hydrogen gas is depleted, although it is intended to allow the resistance to return to an original level in an oxygen environment, the resistance will not ideally return to the original level, and hence a problem has arisen in that repetitive use of this type of sensor itself is difficult to be performed. The reason for this has been believed that since gas detection is performed while the oxide material is being heated, oxidation of the oxide material is advanced by oxygen in air, and hence the resistance does not return to a predetermined initial value after detection of gases is performed, thereby causing drift of the output value.